1. Field of the Invention
This invention relates to a radar-based obstacle avoidance sensor for underground operations. More particularly, this invention relates to an antenna system suitable for use in guided underground directional drilling applications and particularly suitable for use in horizontal directional drilling applications. The system allows for the detection of underground utilities and other buried objects at a distance sufficient to allow the drill to avoid these obstacles. Although particularly suitable for use in directional drilling operations, it is not limited to this application. It may be used in any application where space is limited and sensing of targets in opaque materials is required. The radar design is a novel implementation of a Stepped Frequency Continuous Ground Wave Penetrating Radar designed to fit inside the drill head of a horizontal directional drill or in other applications where space requirements are restrictive. The system consists of the electronics to generate and receive the radar signals, an adaptive antenna designed specifically for various soil conditions, communications and power electronics to allow the radar to be controlled via a single conductor in the drill-string and a Human Machine Interface that performs display, storage and processing functions.
The radar generates continuous wave frequencies typically over a 400 MHz to 1000 MHz bandwidth. This signal is applied to the terminals of an electronically matched antenna that radiates energy ahead of the drill head. The scattered energy from a target, for each frequency, is received by the radar and converted to a digital signal. This data is calibrated and converted to the spatial domain and then transmitted to the Human Machine Interface via the drill string. The Human Machine Interface typically, but not necessarily, provides a simple A-scope radar interface that tracks the targets ahead of the drill. The HMI also interfaces to the drill hardware to allow for automatic shutdown of the drill if utilities, such as water or gas pipelines, are encountered in the drill path.
2. Description of Related Art
Guided directional drilling equipment is being used more often for the installation of underground utilities. These trenchless installations offer significant advantages over trenching operations, including ease of installation in inaccessible areas and lower costs. However, with this installation ease and lower cost comes the potential hazard of cutting existing underground utilities and the significant cost incurred for repair and loss of service. Even with the use of surface locating technology and One Call services, existing utilities are regularly cut. Even though not all utility hits are the result of directional drilling operations, the magnitude of the problem is nevertheless substantial. Companies responsible for cuts are also being charged for revenue loss in addition to repair costs. Hence, to reduce the risk of utility damage it is essential to develop new techniques, other than standard surface locating methods, to locate utilities in the path of and adjacent to new guided drill bores.
Other than standard pipe and cable locators, the most commonly applied geophysical technique to locate utilities is ground penetrating radar. Surveys are normally conducted from the surface and the location and depth to potential utilities are determined from an analysis of reflected energy. Other techniques that have been used include magnetic field sensors, seismic or acoustic techniques, and electromagnetic induction sensors. All these techniques are most commonly applied from the surface and, as such, provide no information as drilling progresses on conditions in the immediate vicinity of the drill. Errors in lateral and depth locations result in utility cuts, both as the drill advances and when the hole is subsequently reamed. According to users, most utility hits occur on the back ream. While utilities are missed as the pilot bore is drilled, they are close enough laterally that they are cut as the reamer is pulled back through the hole. Hence, any drill head technique needs to be able to look both ahead of the drill and to the side. This will assist in the avoidance of utilities directly in its path, when the hole is initially drilled and when the hole is reamed during the product installation phase.
While not applicable in all soil types, ground penetrating radar (GPR) provides one of the fastest and most accurate determinations of target location of any geophysical sensing technique. This rapid data acquisition feature of GPR is essential, since with an advancing drill stem, obstacle avoidance information must be acquired and evaluated rapidly. A means of recording the location of utilities encountered during drilling must be devised. In addition to providing immediate assistance, these data provide input for a database of as-built conditions of pre-existing utilities.
As important as high signal to noise ratio data collection is the presentation of data in a manner that is easily interpreted. Simultaneous rotation and advance of the drill string will result in data that do not necessarily appear the same as that normally collected from surface surveys. Analyses of the expected returns of different utilities at different relative orientations is thus desirable. Because of the rapid advance rate, the drill operator must be able to identify and react to utilities quickly. Alternatively, the drill should be linked to interlock circuits that provide an automatic shutdown if utilities are approached too rapidly for operator intervention. Processing and display must be provided in real time.
There are at least two significant problems associated with conventional antenna systems. The first such problem is non-optimal and/or uncontrollable input impedance properties across the frequency band of interest and a strong dependence of these properties on the electromagnetic constituent parameters of the surrounding medium into which the radiation is directed. The second problem is the inability to effectively concentrate the radiation away from perpendicular to the host, or stated differently, an inability to create a forward-looking radiation lobe.